1. Field of the Invention
This invention relates generally to semiconductor device manufacturing. More particularly, the present invention is directed to methods and apparatus for handling solder balls in forming ball grid arrays (BGAs).
2. State of the Art
Integrated circuit semiconductor devices (ICs) are small electronic circuits formed on the surface of a wafer of semiconductor material such as silicon. The ICs are fabricated in plurality in wafer form and tested by a probe to determine electronic characteristics applicable to the intended use of the ICs. The wafer is then subdivided into discrete IC chips or semiconductor dice, and then further tested and assembled for customer use through various well-known individual IC die testing and packaging techniques, including lead frame packaging, Chip-On-Board (COB) packaging, and flip-chip packaging (FCP). Depending upon the semiconductor die and wafer sizes, each wafer is divided into a few dice or as many as several hundred or more than one thousand discrete die.
Interconnection of discrete semiconductor packages onto a substrate such as a printed circuit board (PCB) is often accomplished with solder preforms having generally a spherical or other shape. In a process using a ball-grid-array (BGA), spherical solder balls are attached to prefluxed metalized locations on a workpiece such as a circuit board or a semiconductor device. The workpiece is then heated to reflow the solder balls, and the solder balls become attached to the metalized locations during subsequent cooling. A semiconductor package or circuit board having a corresponding but reversed pattern of connection sites may then be aligned with the BGA and bonded to it by controlled heating in a reflow furnace.
The use of flip-chip technology with solder bumps has numerous advantages for interconnection, it being widely used in the electronics industry. Flip-chip design provides improved electrical performance for high frequency processor applications, such as mainframes, computer workstations, and personal computers having powerful processors. Ball-grid-array interconnections are of small size. In addition, easier thermal management and reduced susceptibility to EMI and RPI emissions are inherent in the use of BGA technology.
In addition, surface mount technology (SMT) using solder “bump” or ball interconnects eliminates the outer package leads level of interconnection, significantly reducing the cost.
Solder bumps may be formed on a workpiece by processes of evaporation, electroplating, stencil printing, and serial methods. Each of these processes has particular limitations. U.S. Pat. No. 5,672,542 of Schwiebert et al. is an example of a modified stencil printing process.
In U.S. Pat. No. 3,716,907 of Anderson, the use of germanium hemispheres as conductive contacts is disclosed. The germanium hemispheres are connected to the substrates with solder.
Relative to other types of interconnections, the use of solder preforms, in particular spherical or near-spherical balls, has proven to have significant advantages. One advantage is that while the solder balls are formed with ball-to-ball size differences, they may be easily classified by size prior to application to a workpiece. Thus, a uniform size of solder balls may be used within a ball-grid-array.
Various methods have been used for aligning, placing, retaining and fixing solder balls on an array of sites on a workpiece.
In U.S. Pat. No. 5,620,927 of Lee, a template with an array of through-holes is placed on the workpiece and solder balls are introduced into the holes by rolling the solder balls across the workpiece surface. The apparatus may be installed on a tilt table to encourage filling of all holes. In U.S. Pat. No. 4,871,110 of Fukasawa et al., a template having an array of holes is placed on a ball holder with a like array of smaller holes to which vacuum is applied and over which solder balls are rolled. After the array is filled with solder balls, the template and ball holder with balls are removed and the exposed ends of the balls attached to a substrate by e.g., reflow. The template and ball holder are then pulled from the substrate, leaving a ball-grid-array ready for attachment to another substrate or workpiece. A vacuum system is required, and there is no easy way to replace a solder ball onto a bond pad to which a ball did not become attached (i.e., missing ball).
As shown in U.S. Pat. No. 3,719,981, an array of solder balls is arranged on the tacky surface of a pressure sensitive (PS) tape for alignment through a template to solder bumps on a wafer. After thermal reflow, the template and tape are removed.
The use of a template for forming solder bumps or “balls” on a workpiece from flux and solder pieces is disclosed in U.S. Pat. No. 5,492,266 of Hoebener et al.
In U.S. Pat. No. 5,431,332 of Kirby et al., a template is placed over the bond pads of a substrate, solder balls are poured over the template, and an air knife “sweeps” the surface free of excess solder balls.
The use of a ball pick-up tool with an array of vacuum suction ball retainers to pull up balls from an underlying reservoir and place them on a substrate is disclosed in U.S. Pat. No. 5,088,639 of Gondotra et al., U.S. Pat. No. 5,284,287 of Wilson et al., U.S. Pat. No. 5,445,313 of Boyd et al., U.S. Pat. No. 5,467,913 of Nemekawa et al., U.S. Pat. No. 5,615,823 of Noda et al., U.S. Pat. No. 5,680,984 of Sakemi, U.S. Pat. No. 5,685,477 of Mallik et al., U.S. Pat. No. 5,687,901 of Hoshiba et al., and U.S. Pat. No. 5,695,667 of Eguchi et al. It is known in the art that shutting off the vacuum to release each ball onto the substrate is not always successful, and sometimes balls remain attached to the pick-up tool. Again, there is no easy way to replace a missing ball except with a single ball pick-up tool.
U.S. Pat. No. 5,506,385 of Murakami et al. discloses the use of a single manipulable suction head for picking up a solder ball, moving it to a position above a fluxed contact pad on a substrate, and depositing it on the contact pad. Because of the high number of repetitive actions in separate placement of each ball, ball placement is time consuming.
U.S. Pat. No. 5,695,667 shows a single ball suction head which is used to place a solder ball on a contact pad which is missing a solder ball of a ball-grid-array.
The application of flux to solder balls held in a vacuum apparatus by dipping the balls into a flux reservoir is taught in U.S. Pat. No. 5,088,639 of Gondotra et al. and in U.S. Pat. No. 5,284,287 of Wilson et al.
The use of ultrasonic vibration to cause solder ball movement in the ball reservoir, and to remove excess solder balls from a vacuum pick-up tool, is taught in U.S. Pat. No. 5,687,901 of Hoshiba et al.